Gertabelle:
May I ask you again, please, if there is such a definitive statement on attending marriages which cannot be presumed to be valid?
The closest I can find is in one of the print editions of the Baltimore Catechism. I’ll just cut and paste what I posted in two recent threads:
In one of the old Baltimore Catechism print versions (I could not find a link to the version online), it stated that regarding Catholics married by protestant pastors, Catholics could not attend the wedding nor should they give gifts to the couple. If we see the reasoning behind that as those weddings being invalid (perhaps because they involved a person or two people who were divorced and thus ineligible to marry in the Catholic Church, so they would have to go the non-Catholic route), then we can extrapolate from that source that the answer is a clear and resounding “NO!”. Additionally, according to CCC 1868, we share in the sin of others when we do anything to show approval of their actions (not an exact quote, but that is the sense that the CCC 1868 was trying to convey; also see sections regarding scandal).